A new analysis finds that the vast majority of the CO2 emissions associated with America’s carbon intensity decline since the mid-1900s can be attributed to the increasing shares of two energy sources: nuclear fission and natural gas. These two fuels have done more than any others to displace coal and have saved the country 54 billion tonnes of CO2 emissions since 1950 (by comparison, in 2012 the entire world energy sector emitted 35 billion tonnes of CO2).

The Kaya identity, developed by Japanese economist Yoichi Kaya in the early 1990s, states that the total CO2 emissions of an economy can be expressed as the product of four inputs: population, per capita gross domestic product (GDP), energy use per unit of GDP, and the carbon emissions per unit of energy consumed. The third and fourth terms are known as “energy intensity” and “carbon intensity”, respectively.

It is generally acknowledged that in order to reduce CO2 emissions policy makers will need to focus on reducing the latter two terms of the Kaya identity, energy intensity and carbon intensity, rather than on population and GDP. Energy intensity improvements can be achieved by making an economy more energy efficient (more economic output per unit of energy consumption) and by switching from energy intensive industries (such as manufacturing) to less intensive ones. Carbon intensity improvements are achieved by switching from energy sources that emit a lot of carbon (e.g. coal) to ones that emit less carbon (e.g. natural gas) or no carbon (e.g. nuclear, hydro, wind, solar, etc.).

In the United States since 1940 the energy intensity of the economy has declined by nearly 70 percent and carbon intensity by 24 percent, while economy-wide CO2 emissions have nonetheless tripled (from 1.9 billion tonnes CO2 in 1940 to 5.5 billion tonnes in 2011) as a result of population and GDP growth.

But what if the U.S. economy hadn’t shifted from one that burned a lot of coal and oil to one that burned significantly less coal, and used much more natural gas, nuclear power, and renewable energy? In other words, what would the nation’s CO2 emissions be today if the fuel mix had remained at its relatively dirty 1940 proportions? As the graph below shows, keeping carbon intensity constant at its 1940 levels and allowing population, per capita GDP, and energy intensity to progress as they actually did through time, today’s annual CO2 emissions would be 1.7 billion tonnes greater than actual annual emissions. The area between these two lines, which is about 58 billion tonnes of CO2, is the amount of CO2 emissions that we’ve avoided by switching to cleaner energy sources over the last 60 years.

In the 1950s natural gas started to play an increasingly large role in the energy mix, edging out coal in the electric power sector and reducing carbon emissions. In the 1970s nuclear power – which emits no carbon – started to gain share in the nation’s energy supply and in the 1990s renewable geothermal, wind, and solar energy started supplying significant shares of energy. Changes in the shares of petroleum and hydropower in the total energy supply have been relatively small.

What is the relative impact of each of these lower-carbon or zero-carbon fuels in reducing CO2 emissions, relative to the 1940 baseline? To estimate this I assumed that increases in the shares of zero-carbon emission energy sources – nuclear power, geothermal, wind, solar, and hydro – led to proportional decreases in CO2 emissions, relative to the baseline. I discounted the impact that increases in natural gas had on lowering CO2 emissions, because although natural gas is less carbon intensive than the 1940 baseline, it still emits carbon. I used a similar method for petroleum and biofuels, because although these fuels are slightly less carbon intensive than the 1940 economy-wide baseline, they still emit significant carbon emissions.

The results show that the switch from dirtier energy to zero-carbon emitting nuclear power accounted for a full half of the total emissions reductions, or 28.1 billion tonnes over the 60-year period. The switch from dirtier sources to natural gas has had a similar impact, 25.9 billion tonnes. Switching to geothermal, wind, and solar has had a much more modest impact, about 1.5 billion tonnes over the entire period. Other cleaner energy fuel-switching over the 60-year period – which includes changes in the shares of petroleum, liquid biofuels, and hydro ­– accounts for a 3.4 billion tonne reduction.

As more countries get access to modern energy services the world is on track to consume double, triple, or quadruple the amount of energy it does today by the end of the century. This should be celebrated but it means that clean, cheap, and abundant energy sources will need to be made available to limit emissions and avoid the worst consequences of a warming planet. In the context of a “high-energy planet” which will be consuming vastly more energy than it does today, we should expand rather than limit the suite of low- and zero-carbon options available. History shows that in the U.S. context, two technologies that have been publically demonized in recent years – natural gas and nuclear power – have led the charge in decarbonizing the economy and avoiding CO2 emissions. These fuels should not be demonized but rather seen as essential components of a transition to a low-carbon future.

Max Luke is currently pursuing a Master of Science degree in Technology and Policy at MIT, where his research is focused on innovation in the electric power sector. Prior to MIT, Max was a policy associate at the Breakthrough Institute, a San Francisco Bay Area think tank focused on energy, climate, and environment policy issues. At the Breakthrough Institute Max’s research focused on a ...

Fascinting analysis. That said I can't help but feel that by choosing the focus on carbon intensity, you have underplayed the importance of energy intensity, despite noting its huge impact. If you look at the total emissions the US has only had two sustained dips in emissions, in the late 70s and the late 00s.

Oil prices explain most of the first, and pushing half of the second. So while I'm all for making clean energy cheap, there is also good argument for making carbon more expensive, particurlarly in wealthy countries where damage costs are high. Obviously making energy more expensive for poor people is far less palatable.

Thanks, I fully agree that not including energy intensity in the analysis misses a big piece of the emissions mitigation puzzle. I considered doing a similar partitioning for energy intensity, but decided against it for this analysis because it's much more difficult to disentangle the various drivers of energy intensity declines. Drivers include sectoral shifts (e.g. away from energy intensive manufacturing), proportion of GDP spent directly on energy, and energy prices. This paper by Robert Kaufmann provides some insight on how to decompose energy intensity: http://www.sciencedirect.com/science/article/pii/092180099290037S

Thanks for the link! And you're right about untangling the drivers in energy intensity changes. It is a complete nightmare. Offshoring, efficiency, prices, preferences . . . it is hard enough to define them all let alone seperate them

I'm just curious why this post doesn't address carbon emissions from the largest source in the U.S. - oil. While it's definitely true and good that nuclear, natural gas and renewables have helped to reduce carbon emissions from coal, these fuels haven't really made a dent in oil consumption, and thus oil emissions. If the goal is to reduce carbon emissions, doesn't it make sense to go after the biggest offender?

The post does address emissions from oil. As you said, the share of oil in the total US energy mix hasn't changed very much in the last 60 years. Oil energy – as a share of the total – has been remarkably stable through the years. This is indicative that the makeup of the transportation sector – which is the largest consumer of oil products – hasn't changed very much in the last 60 years. Since oil's share of total energy hasn't changed very much, it hasn't contributed to very much of the change in economy-wide carbon intensity of the US. This is why it doesn't show up in my graphs above. Changes in oil, along with hydro, biomass, and biofuels, are all represented by the little grey sliver in the third graph. These energy sources have been quite stable as a share of total energy consumption.

A working figure for natural gas is about 400 g CO2/kwh, for nuclear, depending on the fuel cycle and the type of analysis is between 10 g CO2/kwh - 100 g CO2/kwh, excluding the undoubtedly questionable and oft criticized claims of Storm van Leeuwen, an outlier.

I would note that current fuel cycles, based either on mining and enrichment or Purex reprocessing are hardly optimized for carbon impacts, although the demonization of the world's largest, by far, source or climate change gas free energy has removed much impetus for the scaling of well known superior technologies.

It would follow that natural gas, which is not sustainable in any case, should be replaced with nuclear energy to obtain even better profiles. The opposite is happening regrettably, as evidenced by the recent, and almost certainly environmentally disasterous consequence of things like the announced cloture of Vermont Yankee Nuclear Power Plant. The popular fantasy is that that plant will be replaced by so called "renewable energy." The reality is that it will be replaced by dangerous fossil fuels.

To say that natural gas is better than coal is not to excuse natural gas's very profound climate, safety and mining risks. I note that if one wants to understand gas's climate effects, one needs to look not only at the rising content of CO2 in the planet's favorite waste dump, the atmosphere, but also methane's. Methane is a much more potent GHG, and is in fact, the second largest forcing gas, followed by nitrous oxide.

B&W has about 50 years of experience building small nuclear reactors for the US Navy and big reactors for power companies. Utility nuclear power plants take about 8 years to build; their reactors usually are 1,000 MW, or greater.

B&W has developed a 125 MW nuclear power module that will be built in US factories under controlled conditions to reduce costs and ensure quality. Several modules can be combined to create power plants of 1,000 MW, or greater. The plant can be arranged for water or air cooling of the condenser. The modules use standard 5% enriched U-235 uranium and have a 4.5-year operating cycle between refueling. The modules are fully-assembled and rail/barge-transportable to a plant site.

B&W, seeing the benefits of modularity, is planning to supplement its nuclear module with a fully-assembled, steam turbine-generator module that is rail/barge-transportable to a plant site. It will likely partner with GE for the T-G module

B&W calculates over the 60-yr life of the reactor, each module would avoid about 125 MW x 1,000 kW/MW x 8,760 hr/yr x CF 0.90 x 60 yr x 2.12 lb of CO2/kWh x 1 metric ton/2,204.6 lb = 57 million metric tons of CO2 that would have been emitted by a coal plant.

B&W and Bechtel have formed a joint venture Generation mPower to build the modular power plants. Such standardized plants will be much quicker to license and build and less costly to own and operate.

In June 2011, B&W announced TVA has signed a letter of intent with Generation mPower to build up to six of B&W’s modular nuclear reactors at TVA’s Clinch River power plant site in Tennessee. TVA is seeking approvals from the NRC.

Going modular is a unique opportunity for the US to be in the front of other nuclear industry powers. The sooner the Clinch River plants gets going, the better.

If Boeing can build about thirty $150 million planes per month, then B&W/GE/Bechtel could build about ten $375 million modules per month.

France made a wise decision to go nuclear about 50 years ago. While France will be enjoying low electric rates, its competitors, such as Germany, the US, etc., will be increasing their electric rates, because they need to invest trillions of dollars over several decades to get to France’s low CO2 intensity; a major competitive advantage for France.

- France has about 79% of its power from 19 nuclear plants with 58 reactors, and about 12% hydro. Some of its PWR nuclear plants are designed to be partially load-following, its hydro plants and other plants do the rest.

- France has leading global nuclear companies, such as Areva, GDF-Suez and EDF.

- France reprocesses its “spent” fuel, and that of a few other nations, to make new fuel for nuclear reactors, thereby much better utilizing the uranium and greatly reducing waste. The nuclear fuel burnup is about 5% at the end of a 300-500 day refueling cycle. The other 95% is available for reprocessing.

- France has among the lowest electric rates in Europe.

- France has the lowest CO2 intensity, 0.37 lb of CO2/$ of GDP, of all industrialized nations.

- France built a national, 180-mph rail system that runs on nuclear power.

Except for the first two paragraphs, you're preaching to the converted. You will not find a more enthusiastic person about France's nuclear program than I am. It is, in my view, the very best electrical generation system in the world.

My reading, if I recall correctly, indicates methane sources indicates that the largest sources are probably the result of agriculture - which is also the primary source of the third worst climate change gas, nitrous oxide - followed by, believe it or not, sewage processing.

I don't know anyone who knows how to phase out agriculture, though.

There is, ironically enough, nuclear means to deal with N2O, but they will never generate the popular enthusiasm required, so we'll have to live - or die - with that gas, which is also a very potent ozone destroying gas.

I skated around the N2O issue in a desultory post on another website where I used to write, although much more can be said on the topic: It's a very serious issue.

However the leakage of dangerous fossil fuel methane, irrespective of the issues with agriculture is also serious matter, which is not to say that the other sources are trivial.

I can tell you that if the death toll associated with natural gas - or for that matter oil or coal - were associated with nuclear energy, we'd have an absurd hue and cry, since even when - as was the case with Fukushima - the death toll is trivial (by comparison with gas, oil and coal) we hear it already.

Humanity may deserve what it's going to get.

We will soon experience, according to many scenarios large new sources of methane from decomposing permafrost, from new dams, and possibly most seriously, from the heating of methane hydrates.

Irrespective of the boost that the use of dangerous fossil fuels gave to the world population, I favor their immediate phase out. Nuclear energy - were it to be used wisely, which experience shows it will not be - could make all of them unnecessary; I've convinced myself, if not everyone else, there are no exceptions.

I personally believe it is far too late for nuclear energy to save much of what it might have saved though.